The use of water or a water-based machining fluid has become increasingly important in the EDM art. Such fluids are generally inexpensive and offer relatively high removal rate at some cost of electrode wear. In addition, they are free from the danger of a fire unlike conventional hydrocarbon EDM media such as transformer oil and kerosene. Thus, the practical use of water as the EDM medium first occurred and is now common in traveling-wire (TW) EDM machines where the gap is placed directly in the air and the electrode wear is less significant. The water is commonly deionized water which may incorporate one or more organic components to substantially enhance discharge stability and also to reduce the electrode wear. The capability of wear reduction has now proved the use of such a water fluid to be also advantageous in sinking or generic electrode EDM machines where the tool electrode is a formed or simple electrode to machine a workpiece into a desired three-dimensional shape.
One significant problem arises, however, in the use of a water fluid in EDM processes and is the problem of corrosion or electrochemical attack of the machined surface. This is particularly noticeable when the workpiece is composed of a cemented carbide material such as WC-Co. The cobalt binder has a relatively high electrochemical solubility and thus tends to preferentially dislodge during an EDM operation. As a result, pits or localized irregular formations tend to develop on the machined surface and damage the quality thereof. Such phenomena are generally experienced when the workpiece is a metallic carbide, boride or alloy of high hardness in which a metal of relatively high electrochemical solubility such as Fe, Ni, Cr, Cu, Co, Zn, Al, Cd, Mn, Sn or Sb is a binder or alloy component.
It is known that this problem can be alleviated by providing a power supply circuit arrangement in which each of the machining voltage pulses is of regular polarity with the workpiece being poled positive and the tool electrode being poled negative to cause erosive gap discharges and where each of the machining voltage pulses is followed by an auxiliary pulse of the reverse polarity effected across the gap to make the tool electrode positive and the workpiece negative. While such known arrangements have proven to generally limit the corrosion phenomena, however, it has been found that not only does the alternate polarity reversal reduce the power efficiency, the reverse polarity pulses applied in such a mode tend to adversely affect the regular polarity machining pulses and to unstabilize the resulting erosive discharges.